The “New Covenant” was not a secret. It was prophesied in many places in the Old Testament. The focal point of the “Sacred Secret”  is that Gentiles who believe in Christ as “Lord” would be fellow heirs with Israelites who believe. They would be part of the same Body, a new creation, the “one new man” of Ephesians 2:15, and partakers of “the promise in Christ Jesus.” But does that mean that the only variable was who would make up this Church?
Let us liken the “Church,” which Jesus said he would build out of those Israelites who responded to his message, to a blue caterpillar. The Gentiles, meanwhile, are a red caterpillar, living on a separate tree, despised and looked down upon by the blue one. Using this analogy, was the Sacred Secret that the blue caterpillar (the Jewish believers in Christ) would come over and swallow the red one (the Gentile believers) and become a much bigger and better blue caterpillar? This is the position furthered by some who teach that “Christianity” is really “Messianic Judaism,” and that Gentile believers ought to keep the Jewish feasts, etc. Or was it that the red caterpillar would come over and swallow the blue one? This is the belief that the Church replaces Israel, a belief held by Roman Catholicism, many people partial to Reformed Theology (such as some Lutherans) and a growing number of “charismatic” groups.
Neither of these scenarios is the one set forth in the Church Epistles. The Sacred Secret was that some of the Jews (the blue) and some of the Gentiles (the red) were made by the Lord into a “new creation.” God combined and transformed the old caterpillars into a beautiful purple butterfly, as it were, called “the Church of God,” or “one new man.” The Church of the Body of Christ that began on Pentecost is not in any way an extension of or the continuation of Israel. Rather, it is an entirely new creation that is “neither Jew nor Greek” (Gal. 3:28; Eph. 2:15).
 In most Bibles, the Greek word musterion is translated “mystery,” but it does not mean that which is mysterious or inherently unknowable. It means “secret,” that which is knowable to whom it is revealed, and it should be translated as “secret,” not “mystery,” in Ephesians 3 and many other places.